Radar level gauge systems are in wide use for measuring the filling level in tanks. Radar level gauging is generally performed by propagating an electromagnetic transmit signal towards a product in the tank, and receiving an electromagnetic reflection signal resulting from reflection of the transmit signal at impedance transitions encountered by the transmit signal, including the surface of the product.
The transmitted electromagnetic signal may be radiated towards the product contained in the tank, or may be guided towards and into the product by a transmission line probe. The latter is often referred to as Guided Wave Radar (GWR).
Radar level gauges are often generally classified as either pulsed systems or FMCW-type systems.
Pulsed radar level gauge systems may have a first oscillator for generating a transmit signal formed by pulses for transmission towards the surface of the product contained in the tank with a transmitted pulse repetition frequency ft, and a second oscillator for generating a reference signal formed by reference pulses with a reference pulse repetition frequency fr that differs from the transmitted pulse repetition frequency by a given frequency difference Δf. This frequency difference Δf is typically in the range of Hz or tens of Hz.
At the beginning of a measurement sweep, the transmit signal and the reference signal are synchronized to have the same phase. Due to the frequency difference Δf, the phase difference between the transmission signal and the reference signal will gradually increase during the measurement sweep.
During the measurement sweep, the reflection signal formed by the reflection of the transmit signal at impedance transitions encountered by the transmit signal is being correlated with the reference signal, to form a measurement signal based on a time correlation (equivalent to a phase relation) between the reflection signal and the reference signal. Based on the measurement signal, the filling level can be determined.
In FMCW-type radar level gauge systems, a transmit signal with a time-varying frequency is transmitted towards the surface and the distance to the surface is determined based on the frequency (and/or phase) difference between the transmit signal and the reflection signal. The transmit signal and the reflection signal may be combined in a mixer, which results in a so-called intermediate frequency signal indicative of a phase relation (specifically a phase/frequency difference) between the transmit signal and the reflection signal. The distance to the surface can be determined based on the measured phase/frequency difference and the known variation over time of the phase/frequency of the transmit signal.
In order to determine the filling level of the product in the tank, the distance to a reference impedance transition at a known vertical position is often determined in addition to the distance to the surface of the product. The filling level can be deduced from the distance between the reference impedance transition and the surface of the product, and the known vertical position of the reference impedance transition.
The reference impedance transition may, for example, conveniently be provided through an impedance discontinuity at the connection between the signal propagation device (such as antenna or transmission line probe) and the transceiver of the radar level gauge system.
In this case, for example, the reference echo signal reflected by the reference impedance transition may have considerably larger amplitude than the surface echo signal reflected by the surface of the product.
This may make it difficult to determine a suitable gain function for the received reflection signal and/or determine a suitable gain function for a measurement signal formed based on the reflection signal and the transmit signal. Either the gain may be too high, which may result in the reference echo signal being saturated, or the gain may be too low, which may result in difficulties in distinguishing the surface echo signal.
U.S. Pat. No. 6,690,320 addresses this problem by using a relatively low amplification factor in the beginning of each measurement cycle, and once timing information has been extracted from the fiducial pulse, the software increases the amplification factor to a higher value which is optimal for extracting the timing information from the much smaller level echo pulse.
However, there appears to be room for improvement in relation to the method according to U.S. Pat. No. 6,690,320, in particular for cases where the distance between the reference impedance transition and the surface of the product is relatively small.